The following description of the background of the invention is provided simply as an aid in understanding the invention and is not admitted to describe or constitute prior art to the invention.
The present invention relates generally to the field of automated testing of semiconductor devices. Specifically, the present invention relates to a system and method for controlling the temperature of semiconductor devices in an automated testing handler.
Control of DUT (“device under test”) temperature in testing operations has been practiced for some time. For example, U.S. Pat. No. 5,297,621 discloses a liquid bath in which devices are immersed during testing. The liquid in the bath is inert and comprises a mixture of two liquids having boiling points above and below the desired temperature. By varying the mixture of the two liquids, the liquid in the bath is arranged to have a boiling point which is at the desired operating temperature of the DUT (“set point temperature”). Heat generated by the DUT is dissipated by convection within the bath and by nucleate boiling of the liquid on the DUT. Heat transfer from the DUT to the liquid is facilitated by placing a heat sink in contact with the DUT.
Soaking refers to the process of taking a device under test (DUT) from an ambient temperature (typically 17-23° C.) to a test set point temperature (typically between −55° C. and 150° C.). The purpose of soaking is to bring each DUT to a set point temperature and stabilize the DUT's temperature at the set point temperature as quickly as possible prior to the start of test (SOT) of the DUT such that the DUT is at a stable temperature within a guard band of the set point temperature.
FIG. 1 illustrates time vs. temperature readings for a step-wise soaking method. The time, in seconds, is displayed on the X axis in two (2) second increments. The measurement of temperature for a heater for heating the DUT and the temperature of the DUT is represented along the Y axis. As shown in FIG. 1, the total soak time of a DUT in the system begins when a DUT is picked up by a chuck of the automated testing handler. At 0 seconds, a DUT is picked up by the chuck of the automated testing handler. A heater is activated or is active at time zero. The temperature of the heater increases until the temperature of the heater has reached an overdrive (OD) temperature in a guard band range of temperatures. The guard band temperature range is user configurable and is a range around the desired set point. For example, as shown in FIG. 1, if the soak temperature is 100° C. with a guard band of +/−5° C., the range would be from 95°-105° C. Overdrive temperatures are typically 3-15° C. above the set point temperature, but are very dependent on the set point temperature itself, the physical construction of the device, and the quality of the thermal interface between the device and the chuck. It should also be noted that for cold set points (below ambient), the overdrive temp would be below the set point. The period of time the heater enters and remains at the OD temperature in the guard band is known as the OD time. Next, as shown in FIG. 1, at an “index time” the DUT having an ambient temperature is socketed. After socketing, the DUT is heated by the heater which is driving toward an OD temperature.
As shown in FIG. 1, once the heater reaches a temperature in the OD temperature band a period of time referred to as “OD time” begins. During OD time, the temperature of the heater and the socketed device is driven higher than a temperature at which the DUT is to be tested. Next, at a specific time triggered by the automated testing handler, the temperature of the heater decreases to that of a hold temperature. The hold temperature is in a hold temperature guard band where the temperature is generally very close to, if not at the set point temperature. So for a 110 set point, the hold temperature could be 110 or slightly higher to account for temperature loss to ambient. Note that for cold set points, the hold temperature would be (most likely) below the set point.
As shown in FIG. 1, the period of time the heater enters and remains in the hold temperature guard band is referred to as the hold time. Accordingly, the temperature of the DUT decreases. As shown in FIG. 1, at the end of a predetermined period of time, the temperature of the DUT falls within the hold temp guard band at the hold temperature. The hold temperature is also the test temperature and at this predetermined period of time, the automated testing handler begins testing the DUT. In the alternative, at the expiration of the hold time, if the hold temperature of the DUT does not equal the predetermined test temperature, the automated testing handler causes the heater to transition to the test set point temperature which in turn drives the DUT temperature to the predetermined test temperature.
One drawback to the step-wise soaking method illustrated in FIG. 1 is that in order to stabilize a DUT at the test set point temperature, the temperature of the DUT must initially be driven beyond the set point temperature so that the temperature of the DUT will settle back to the test set point temperature. Ideally, an optimum soak method would drive the temperature of a DUT directly to the set point guard band without the need to first raise the temperature above the set point.
In another system for soaking DUTs, the actual temperature of the DUT is used as feedback information to control the soak method. In order to obtain the actual temperature of the DUT, the DUT is provided with a temperature sensor. For example, an internal thermal diode or resistance temperature detector (RTD) may be mounted onto a DUT. The purpose of the internal thermal diode and RTD is to provide the automated handling system with instantaneous information related to the temperature of the DUT during the soak process. This direct temperature feedback (DTF) method is illustrated in FIG. 2. FIG. 2 shows plots of time vs. temperature for a heater controlled by the automated testing handler and the temperature of the DUT. As shown, the DTF method reduces the total soak time to less than three (3) seconds. At 0 seconds the DUT is picked up by a chuck. Immediately after time=0, the heater, based on temperature feedback information provided by the DUT's temperature sensor drives the DUT temperature directly to a set point temperature. Accordingly, at less than three (3) seconds the DUT is ready to be tested by the automated testing handler.
Unfortunately, in normal production, it is not feasible to electrically connect a temperature sensor to a DUT while the DUT is being transported around in the handler. Thus, a soak method that approximates the DTF method is desired for practical use. It is thus an object of the present invention to provide a method and apparatus which allows accurate and efficient temperature control of devices during soaking.